Properties Of Jets In High Energy Heavy-ion Collisions

The hot and dense quark-gluon plasma(QGP)predicted by Quantum Chromodynam-ics(QCD)can be produced in high-energy heavy-ion collisions.High transverse momen-tum partons propagating in the QGP will have strong interactions with the medium,lead-ing to parton energy loss and transverse momentum broadening,a phenomenon called "jet quenching".Effects of jet quenching will result in the suppression of the yield of final state hadrons or jets in nucleus-nucleus collisions compared to proton-proton collision.This is observed in heavy-ion collision experiments at the Relativistic Heavy-ion Collider(RHIC)at Brookhaven National Laboratory(BNL)and Large Hadron Collider(LHC)at CERN,and is considered as one of the evidences of the formation of the QGP.In order to investigate the properties of the QGP and extend our understanding of strong interaction via jet quenching,we have developed a linear Boltzmann transport(LBT)Monte-Carlo model to simulate high-energy partons’ propagation and multiple interactions in the QGP.This model includes 2→2 elastic scatterings and medium modified gluon radiation processes based on perturbative QCD,and can track both initial shower partons and jet induced medium recoil partons.We find that in a static medium high-energy partons will transfer energy to low-energy medium recoil partons.Parton energy loss has a quadratic dependence on the propagation length at the early stage but becomes saturated later on due to energy conservation.For a jet with a given cone size whose phase space includes some low-energy recoil partons,its energy loss is smaller than that of a single parton and main-tains a quadratic dependence for a longer propagation path.This indicates that thermal recoil partons induced by jets from the medium have a non-negligible contribution to the jet energy loss.In phenomenology,we have calculated the nuclear modification factor RAA for the in-clusive jet in Pb+Pb collisions at LHC within the LBT model,in which the evolution of the bulk medium is described by the event-by-event(3+1)D hydrodynamic model.Despite the final hadron density at mid-rapidity at(?)=5.02 TeV is about 20%larger than that at 2.76 TeV,the measured inclusive jet RAA at both colliding energies are very similar.Our study shows that jet RAA depends not only on the in-medium jet energy loss due to jet quench-ing,but also the initial jet spectrum in proton+proton collisions.Specifically,the initial jet flavor composition,radial expansion of the medium and the inclusion of the jet-induced medium excitation all conjoin to contribute to the observed phenomenon.In the mean-time,we also investigate the anisotropy of the inclusive jets within the LBT model,which can also well describe the experimental data.We find there is an approximately quadratic correlation between jet and bulk anisotropy.To phenomenologically study jet energy loss in detail,we employ the state-of-art Bayesian analysis on the experimental data of the inclusive jet and photon tagged jet to extract the distributions of jet energy loss.We find that the averaged jet energy loss has a dependence on the initial jet momentum that is slightly stronger than a logarithmic dependence.The extracted scaled jet energy loss distribution has a large width and indicates that jets only encounter a few number of out-of-cone scatterings.These properties are consistent with calculations within the LBT model.Furthermore,we point out that pT broadening due to jet quenching can lead to a drift of jets along the direction of the gradient of the jet transport coefficient q perpendicular to the jet propagation.Such a drift will lead to an asymmetry of the transverse momentum which depends on the spatial distribution of q and jet propagation path.This phenomenon can be studied by solving a simplified Boltzmann transport equation.In high energy heavy-ion collisions,we can select a plane defined by the initial beam and a tagged photon within the LBT model,and find the asymmetry of the transverse momentum with respect to such plane closely relates to the initial production position of jets.This gradient tomography can be used to localize the initial production of jets and therefore to investigate jet quenching and the properties of the QGP in detail.